Aircraft gas turbine engines are typically supplied with lubricant from a pump driven lubricant supply system. In particular, the lubrication supply pump, which may be part of a pump assembly having a plurality of pumps on a common shaft, draws lubricant from a lubricant reservoir, and increases the pressure of the lubricant. The lubricant is then delivered, via an appropriate piping circuit, to the engine. The lubricant is directed, via appropriate flow circuits within the engine, to the various engine components that may need lubrication, and is collected in one or more recovery sumps in the engine. One or more of the pump assembly pumps then draws the lubricant that collects in the recovery sumps and returns the lubricant back to the reservoir.
In many instances, the pump assembly pumps are implemented as positive displacement pumps, which are driven by the engine via an interposed gearbox assembly. Thus, the speed of the pumps is directly proportional to the rotational speed of the engine. As a result, lubricant flow rate to the engine is controlled solely based on engine speed. However, the lubrication needs of the engine may also vary with other parameters, not just its own rotational speed. For example, the engine lubrication need may vary with engine load and the speed variations, with lubricant temperature, and with external pressure and temperature, which vary with aircraft altitude.
In view of the foregoing, many aircraft gas turbine engine lubrication supply pumps may be designed to supply lubricant to the engine under certain specified design conditions, which may be, for example, the most unfavorable operating condition expected. For example, the supply pump may be designed to supply design intent flow at maximum aircraft altitude, and highest expected lubricant temperature. This approach may result in an over-sizing of the pumps, and thus excess lubricant flow, when conditions differ from the design conditions. Typically, this excess lubricant flow is controlled by implementing a recycle control system, in which a pressure regulating valve downstream of the lubricant supply pump bypasses excess lubricant flow back to the suction side of the pump.
Although the above-described lubricant supply system is generally safe, reliable, and robust, it does suffer certain drawbacks. For example, because the lubricant pumps are over-sized, the system piping circuit may also need to be over-sized, which can increase overall system size and weight, the pumps may needlessly dissipate energy at many operating conditions, and/or excess lubricant may be supplied to and present in the engine.
Hence, there is a need for an aircraft engine lubricant supply system that does not use over-sized pumps and/or system piping, and/or that does not needlessly dissipate energy at many operating conditions, and/or does not supply excess lubricant to the engine. The present invention addresses one or more of these needs.